Optical data systems rely on a beam of light to write and read information on a storage medium. There is a fundamental constraint—the diffraction limit—on how tightly the beam of light can be focused. This limit is directly related to the wavelength of the light. With a shorter wavelength, a smaller spot of light can be made and hence, more bits can be stored on a given area. The resolution limit is given by Abbe's equation:Resolution=(Wavelength*0.61)/(Numerical Aperture).
Reduction of the size of the lasers spot requires the use of a shorter wavelength laser and/or a higher numerical aperture lens. In going from CD to DVD to DVD-Blu-Ray, the numerical aperture was increased and the laser wavelength was decreased from 0.55 at 780 nm, to over 0.6 at 635 nm, and finally to 0.85 at 405 nm. Shorter wavelength light, as well as lenses of a higher numerical aperture (NA), produces focused spots with a smaller depth of focus (DOF) compared to systems using a longer wavelength and lower NA optics. The DOF can be expressed as follows:DOF=2*Wavelength*(Refractive Index)/(Numerical Aperture)2.
Using optical far field techniques sub-diffraction-limit marks cannot be detected on a storage medium. In the most advanced DVD-Blu-Ray technique utilizing a NA of 0.85 and a wavelength of 405 nm, the smallest mark size is 140 nm. In addition, keeping the focal point at the right position with respect to the medium to record and read marks requires substantial effort by the servo system, since any disc wobbling and other imperfections during operation disturb the focal position.
There is a need to achieve an increase in data capacity in optical storage systems beyond that of DVD-Blu-Ray.